If you're building almost anything that uses metal, from a nuclear submarine to a laptop computer, one thing you'll need to be able to do is join
metals together.
Welding is a way of tightly bonding
two metals by melting them where they meet, while soldering involves
making a joint between components in an electric or electronic
circuit. Both are highly effective, though they're very different and
work in completely different ways—don't mix them up!
Photo: Arc welding uses electricity to generate intense heat that fuses metals together. It generates very bright light as well as intense heat; that's why welders need to wear protective masks. Photo by Joshua C. Kinter courtesy of US Navy and
Wikimedia Commons.
You can't really join metals with adhesive—not
with ordinary glue, anyway. But you can join them by melting them together in a process known as
welding. The basic idea is simple: you apply
a source of heat to melt the two metals so they fuse and form a secure joint.
Usually (though not always) you add other materials as you apply the heat:
a filler (an extra piece of metal,
supplied from something called a welding rod, which seals
up any gaps where the main metals meet) and a flux (a
nonmetallic chemical that helps to stop the molten metals forming oxides and nitrides with
gases in the air, which weakens the joint). As an alternative to
using a flux, you can weld in an atmosphere from which the air has
been removed (filled with other, nonreactive gases such as argon, for
example).
Photo: A welder works on a pipe during construction of the Douglas Dam
in Tennessee, which was built in record time in 1942. Credit: Alfred T. Palmer, Office of War Administration,
Library of Congress,
Prints and Photographs Division.
Most forms of welding involve joining metals with heat alone. But
they differ in where the heat comes from. One common form of welding involves using
an oxyacetylene gas torch, which makes an intense flame by burning
acetylene (an energy-rich fuel made from a simple hydrocarbon molecule) in a rich
supply of oxygen. Although convenient and portable,
oxyacetylene torches are relatively expensive to use (because the
fuel is supplied in gas cylinders).
Photo: A welder lights up an acetylene torch.
Photo by Dugan Flynn courtesy of US Navy and
Wikimedia Commons.
In factories, it's usually more
convenient to weld with electrical
power using a technique known as arc welding. Instead of a
gas torch, you use a piece of metal called an electrode
connected to a high-current power supply (hundreds of times higher than the
ones that flow through appliances in your home). As you bring the
electrode up to the joint you're welding, it creates a spark or arc
that melts the metals together. Arc welding produces both bright
visible sparks and discharges of ultraviolet light, both of which can
lead to blindness; that's why you'll always see people arc welding
behind wraparound protective visors. Other heat sources for precision
welding include ultrasonics, lasers, and electron
beams.
You can also weld materials by forcing them together through sheer
pressure, with or
without extra heat. This is known as pressure
welding; used for many hundreds of
years by blacksmiths and other artisans, it's
one of the oldest metalworking techniques. The basic process involves
heating metals in a forge and then hammering them together so they fuse.
One way to make arc welding safer is to get an industrial robot to do it for you. Car bodies have been welded by robots for decades. The first welding robot, the
Unimate, made its debut in a General Motors plant in 1961.
Photo: Left: A typical car-welding robot. The robot (made by Kawasaki) has an electrical welding gun
(made by Nimak) attached to the end of it and it's about to complete a weld on the white car body on the right.
This is called spot welding: the robot arm moves the gun to the precise spot on the car that needs welding and a large but brief electric current leaps between the electrodes to complete the weld.
Right: A closeup of the welding gun showing the two electrodes, which are usually made from a copperalloy.
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What is soldering?
Photo: Although welding and soldering can look similar, they do completely different things. Top: This welded joint mechanically fixes two pieces of metal together on a wheelchair footplate. Bottom: These soldered joints on the back of a printed circuit board make a reliable electrical connection between the components and the tracks on the board that join them together.
Soldering looks similar to welding—but it's quite different! In
welding, you're trying to make a super-strong joint between two pieces of metal. Often a welded joint has to stand up to incredible stresses and strains—for example, if
you weld parts of a car body or an airplane fuselage together. So the
objective is to make a good mechanical
connection. When you solder, the idea is usually to to make a good electrical connection.
Solder looks a bit like an unwrapped paperclip, though it's much
softer, and it generally comes in tubes and reels. It's an alloy of
different metals that has a relatively low melting point. The solder I
use, which is typical, is made of
99.25 percent tin and 0.75 percent copper,
though other metals such as zinc, silver, and bismuth are also used.
(Lead was once widely used in solders with tin, but has now been largely phased out
for safety reasons). Solders sometimes also contain fluxes to
prevent the formation of oxides.
Why do you need to solder? Electronic
circuits are made of discrete components: tiny devices such
as resistors, capacitors,
transistors, and LEDs
that do specific
jobs. When you put them together in different ways, you can build all
kinds of amazing electronic gadgets, from radios
and televisions to
calculators and computers. The components all have
little metal
legs—terminals that you use to connect them into the circuits. You could
just wire these legs together with electrical cables, but the wires might
drop off or wriggle free and the connections wouldn't be reliable, so anything you
built this way wouldn't work very well. And that's where solder comes
in: it makes a much more effective electrical connection.
Photo: Solder looks and feels like a length of paperclip that's been unwrapped and then coiled up in a plastic dispenser tube like this one. You pull out a short length as you need it. This is lead-free solder made mostly from tin and copper.
If you want to make a good soldered joint, you don't solder straight
away. First, you clean
the components you want to join (for example, by scraping them with a
knife to remove any surface oxides). Then you make a good mechanical
connection between them (by wrapping the cable tightly round the
component or whatever). Only then do you make a good electrical connection by melting some solder on top.
Photo: A typical soldering iron—the one I've owned since I was about 14. A soldering iron is essentially just a heating element powered by electricity that has a robust outer case (the bit) designed to withstand endless heating and cooling. Bits are designed to be interchangeable and come with wider or narrower ends for work of different precision. The other essential component (top right) is a stand where you can safely rest your iron though, if you look closely, you'll notice that this iron has a curved clip on it (just above the black rubber finger grip) for resting or propping it up. Since I'm using a UK electrical supply, I've fitted a red indicator light to the plug (top left) to help me remember when the iron is switched on, for added safety.
How does it work in practice? You melt the solder over a joint by
applying a hot tool called a soldering iron
(essentially a hot
piece of metal with a pointed tip, with the heat generated inside it
by an electrically powered heating element). It's very important to
note that solder is not glue: it is not
designed to make a mechanical connection. If you rely on solder alone to fasten two
wires together, they'll probably break apart sooner or later. It's
important to make a good mechanical connection and then solder on
top. There are good and bad ways to solder, some of which make poor
joints that don't conduct electricity properly. (For example, if you
move a soldered connection while the solder is still molten, you will
generally get a badly formed or cold joint,
which will be dull-colored, irregular, and pitted.) If you plan on
doing your own electronic projects, the first thing to do is learn
how to solder properly. You'll find a couple of handy demonstration videos in the "Find out more" section at the end
of this article.
Photo: Here's my soldering iron in action. You solder by holding your hot soldering iron to the joint in your circuit where you want to make an electrical connection. Then, with your other hand, you apply the solder until it melts in a blob on top of the joint, usually with a puff of "smoke" (actually the metals in the solder turning into gas form).
Other uses of soldering
What I've said above applies essentially to electrical and electronic work, but there are other kinds of soldering too.
Plumbers use solder to join copper pipes together in a permanent (but still reversible) way that creates a
mechanically robust and waterproof joint. The process is broadly similar to electrical soldering, although the type of
solder is chemically different and generally you'll work with a handheld gas torch. Still in plumbing,
and closely related to soldering is a process called sweating, in which you allow solder to melt onto, and suck into,
a heated pipe joint.
Soldering is also used in the jewelry business to join fine pieces of metal together
(to create brand new decorative pieces or repair old, broken ones). Again, the concept is broadly similar to electrical soldering—you're using a molten alloy to join two pieces of metal securely together—but the process is different: you use a gas torch and much higher temperatures than you'd use for electrical soldering or plumbing. High-temperature soldering like
this is sometimes called hard soldering or silver soldering
to differentiate it from lower-temperature soft soldering (for electrical and plumbing work);
confusingly, within silver soldering, you'll find different solders described as
hard, medium, easy, and extra-easy (reflecting how easy it is to melt them
rather than how simple they are to work with—so "extra-easy" melts at the lowest temperature).
For comparison, here are some typical solders and rough melting temperatures for electrical, plumbing, and jewelry work:
Work
Solder
Temperature
Electrical (soft)
99.3% tin, 0.7% copper
~227°C or 440°F
Plumbing (soft)
97% tin, 3% copper
~200°C or 400°F
Jewelry (hard)
76% silver, 21% copper, 3% zinc
773°C or 1425°F
What is brazing?
Brazing is a similar process to soldering
except that the filler you use (an alloy, equivalent to solder) works much more like an adhesive. When you
solder, you're simply melting a low-temperature alloy on top of the terminals you're joining together to make a reliable electrical bridge between them. The terminals themselves don't actually melt
(unlike in welding) and aren't usually changed in any way: the filler just sits on top.
But when you braze, you work at a much higher temperature (maybe 450°C or 840°F).
The filler melts and seeps right into the surface of the metals you're joining, so
it binds them together securely. Brazing is thus a bit like a cross
between welding and soldering and it's mainly concerned with
making a secure mechanical joint.
Brazing can be used to join many different metals
and alloys, and it's widely used by plumbers and gas engineers to make smooth, neat, robust,
leak-proof joints in water and gas pipes.
Photo: Brazing a metal joint. A gas torch is used to heat the two metal pieces that are being
joined. Once they're hot enough, a brazing rod (the filler, equivalent to solder) is moved along so it
melts on the hot metal and "feeds" into the joint. Photo by Joshua Mortensen courtesy of
US Navy.
Soldering by Jonathan Hare, Vega Science Trust. In this short video, Jonathan shows how to solder a component to a circuit board and how to tin wire before you start.
How to solder: by Mike Allen, Popular Mechanics. Mike gives an excellent, brief demonstration of soldering here, taking pains to point out why you need a good mechanical joint before you apply your solder to make an electrical connection.
Make: Electronics by Charles Platt. Maker Media, 2015. This friendly, hands-on beginner's guide to electronics introduces soldering in "Experiment 12: Joining Two Wires Together" (p109 of the Second Edition) and includes a real-world example use: how to shorten a power cord.
Principles of Soldering by Giles Humpston and David M. Jacobson. ASM International, 2004. Another detailed guide that covers soldering, welding, and brazing.
Principles of Brazing by David M. Jacobson, Giles Humpston. ASM International, 2005. This guide focuses on the detailed metallurgy of brazing.
Articles
Soldering Safety: A useful risk assessment from Cambridge University's Department of Engineering and a handy summary for teachers and others who have to anticipate the safety issues of letting students loose with soldering irons.
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